The present disclosure relates to a needle assembly for injecting a small amount of a chemical, such as vaccine, into a skin upper layer near a skin surface, particularly into a target portion in a dermal layer near an epidermal layer, and a syringe including this needle assembly for injection of a chemical into the skin upper layer.
A chemical such as influenza vaccine or other biologicals is generally injected into subcutaneous tissue of skin, or injected into muscle tissue located at a region deeper than subcutaneous tissue for intramuscular injection. It has become known in recent years that a dose of vaccine decreases when vaccine is injected into a target portion of a skin upper layer containing a large number of immunocompetent cells, such as the epidermis and dermis.
Skin is constituted by an epidermal layer located at the surface of the skin and having a thickness ranging from 50 μm to 200 μm, a dermal layer continuing from the epidermal layer and having a thickness ranging from 0.5 mm to 3.5 mm, and a subcutaneous tissue layer located at a portion deeper than the dermal layer. Immunocompetent cells such as Langerhans' cells present in stratum spinosum epidermidis within the epidermal layer and a large number of dendritic cells within dermis present in the dermal layer near the epidermal layer are dendritic, and have receptors having a skin immunity function to react to outside antigens.
The epidermal layer is an extremely thin layer located on the surface of skin and having a thickness ranging from only 50 μm to 200 μm from the skin surface. In addition, the epidermal condition and thickness considerably differ depending on ages such as adults, juveniles, children, and infants, and further on genders, individuals, and disease and treatment histories. Accordingly, it is extremely difficult to inject an extremely small amount of vaccine, i.e., an amount of 200 μL or smaller, selectively into stratum spinosum epidermidis containing Langerhans' cells of these persons subjected to vaccine injection by using a needle that is considerably larger than the layer thickness of the epidermal layer for reasons of strength and manufacture of the needle.
It is expected that immunocompetence can be improved and a vaccine dose can be reduced if the vaccine is injected selectively into a skin upper layer corresponding to an epidermal layer and a dermal layer and located near the stratum spinosum epidermidis containing Langerhans' cells or the dermal layer containing dendritic cells within the dermis.
The skin upper layer contains a large volume of elastic fibrous tissue such as elastin and collagen, and therefore is harder than subcutaneous tissue. In this case, pain may be generated by injection of a large amount of a chemical into the skin upper layer. Accordingly, it is necessary to reduce the amount of the chemical injected into the skin upper layer to an extremely small amount, such as 200 μL or smaller, preferably 100 μL or smaller.
For meeting this necessity, there has been investigated development of an efficient syringe capable of diffusing an extremely small amount of a chemical toward Langerhans' cells, and further injecting a vaccine selectively into a dermal layer located near epidermis and containing a large number of dendritic cells within the dermis to form a wheal at which the vaccine is to be maintained for a period of time.
For example, there is known a syringe constituted by a cylinder body for storing a chemical, and a needle assembly including a needle, and developed for injecting a small amount of a chemical into any part of a dermal layer of a skin upper layer. The needle assembly is separated from the cylinder body, and attached to a chemical discharge tube located at a tip of the cylinder body immediately before injection.
For example, JP 2011-212185 A discloses a chemical injection device including: a cylinder body which includes a liquid chamber storing a chemical; a pusher which is constituted by a gasket attached to a plunger and shifting inside the cylinder body; a cylindrical holding portion provided along a cylinder body outer circumference; a connection portion which connects the cylinder body and an end of the holding portion; and a needle assembly that includes a needle hub for holding a needle and injects a chemical from a needle tip slightly projected from the needle hub into a skin upper layer. A chemical discharge tube disposed at a cylinder body tip is pressed into a fitting hole of the needle assembly to be attached to the needle assembly.